OCR Text |
Show humic acid. Vanadium may be found in trace amounts; however, it is not considered a major element. Alumina and silica are the dominant impurities found in coal ash. They exist primarily as quartz and clays, as shown in Table 5. With the addition of titanium, this group is usually referred to as the acidic constituents in coal ash. The melting temperature of the group is high (i.e., +2800°F). During combustion at moderate temperatures, they remain solid, inert particulates and thus, when treated by themselves, are considered innocuous with regard to fireside deposits. The remaining elements are referred to as the basic constituents in coal ash. This group includes iron, calcium, magnesium, sodium, and potassium. When iron, calcium, and magnesium are isolated, they have high melting temperatures like alumina and silica and generally do not cause serious problems. Unlike alumina and silica, they do react with sulfur in the flue gas at low temperatures to form sulfates. When blended with the acidic ash, the basic constituents (individually or as a group) act as a fluxing agent and lower the melting temperature substantially. FUSIBILITY OF ASH The fusibility of the ash is primarily responsible for furnace slagging. It can contribute to convection pass fouling if the temperature in the furnace exit (separating the radiant portion of the steam generator heat transfer from the convective heat transfer) is improperly selected or alternative fuels with decidedly different fusibility characteristics are fired. Historically, the fusibility of the ash has been based on the composite analysis of the coal sample. 1-14 |